1. Field of the Invention
The present invention relates to a welding power source and, more perticularly, to a welding power source for an arc welding system utilizing an industrial robot.
2. Description of the Prior Art
In FIG. 7, an example of conventional arc welding system unitizing an industrial robot such as being proposed in Japanese Laid-Open Patent application No. H2-179361 is shown. This welding system Wc includes a robot body 700, a control unit 100 for the control of the robot 700, a wire contact detector 2600, and a welding power source unit 200. The robot body 700 has a welding torch 600 for supporting and feeding a welding wire 400 in the correct position above a work 900 to join thereof.
In this type of arc welding system, a DC voltage (not welding power) applied between the welding wire 400 and the work 900 greatly reduces when the wire 400 contacts with the work 900. Such voltage reduction is detected by the wire contact detector 2600, and is compared with a reference voltage by an comparator 1700 (FIG. 8). Based on this comparison result, a "wire contact" representing such a condition on which the wire 400 contacts with others can be detected.
Utilizing thus detected "wire contact" condition, it is possible to sense conditions of "work position", "wire sticking", and "torch contact". "Work position" represents the degree by which the work 900 is out of the correct welding position. "Wire sticking" represents such a condition on which the welding wire 400 stuck to the work 900 even though the welding operation has finished. "Torch contact" represents such a condition on which the welding torch 600 contact with the work.
The welding power source unit 200 produces a welding power by transforming three phase power supplied from the externals. These transformed power is further rectified by diodes 2000. The output of these diodes 2000 are connected to the robot 700 by lines 2100 and 2200 via relay CRb. This relays CRb is controlled by the controller 100 that is also connected to the detector 2600 by a line 2400 and to the robot 700 by a line, as best shown in FIG. 7.
An internal resistance 1300 having one end connected to the line 2200 and other end connected to a select switch 2901 incorporated in the detector 2600 by a line 2300 is provided. The select switch 2901 selectively selects either one of line 2300 and a line connected to a high DC voltage source 2900 also incorporated in the detector 2600 based on a command given by the controller 100 through the line 2400. Thus produced welding power is supplied to the robot 700 through lines 2100 and 2200. Relay CRb are inserted in the lines 2100 and 2200 before the internal resistance 1300. This relay CRb is actuated by the controller 100 through a signal line 2650.
In FIG. 8, the construction of wire contact detector 2600 is shown schematically. A positive voltage of the DC power is applied to the output line 2100 on a cathode side of diode 2000, wherein this line 2100 is referred to as "torch cable" hereafter. The voltage detected on the torch cable 2100 varies, depending on whether the welding wire 400 contacts with the work 900, or not. The difference of voltage between when the welding wirer 400 contacts with the work 900 and when welding wire 400 does not contact with the work 900 is detected as a "detection voltage." This detection voltage is compared with the reference voltage by the comparator 1700 for sensing the above described conditions.
The controller 100 includes a robot control unit 2500 incorporated therein for controlling the robot 700. The robot control unit 2500 has a wire contact detector switching relay 1100, a wire contact detection start relay 1200, and welding start relay 1300. The relay 1100 is connected to relays MS and CRb1 in the detector 2600, and closes on receipt of a wire contact detection circuit switching signal Ssw from the controller 100. The relay 1200 closes on receipt of a wire contact detection start signal Swc from the controller 100. The relay 1300 is connected to a relay CRb in the detector 2600, and closes on receipt of a welding starting signal Sws from the controller 100. The robot control unit 2500 also watches the voltage between a relay CRa1 in the
The welding power source unit 200 includes the internal resistance 1300 connected between the torch cable 2100 and a base metal cable 2200, wherein the internal resistance 1300 is a bleeder and is functioning as a welding voltage detector. When a DC voltage is applied with the torch cable 2100 with the wire 400 that is not in contact with the work 900, an electric current flows through the resistance 1300. In this time, the DC voltage is divided by resistances 1300 and 1400, and thus divided voltage is input to the comparator 1700.
However, when the wire 400 is in contact with the work 900, the both ends of internal resistance 1300 are connected to each other through the torch cable 2100, work 900, base metal 800, and base metal cable 2200 each having impedance. Therefore, the DC voltage is divided by the internal resistance 1300, a parallel resistance component of the torch cable 2100 that is short, and the resistance 1400, respectively. Thus divided voltage at a Junction point 1500 is input to the comparator 1700. As a result, the voltage input to the comparator 1700 is different from that when the wire 400 and work 900 are not in contact with each other. This difference in input voltage causes the comparator 1700 to output different results. In consideration of this, an adjustable resistor 1600 is provided for adjusting a threshold of the comparator 1700. This threshold corresponds to the reference voltage of the comparator 1700, as described above.
However, the lengths of the torch cable 2100 and the base metal cable 2200 are variably determined according to the applicable conditions, and are not fixed. Also the surface condition of the work 900 are subject to the presence and quality of surface treatment or oil film coating. The ambient temperature of these components are not constant. These factors affect the impedance of the components. As a result, the overall resistance of those components are not stable.
In addition to this, the internal resistance 1300 that is usually comprised of a resistor having about 100 ohm resistance is subject to the ambiences. The input voltage for the comparator 1700 varies furthermore.
On the other hand, the current flowing when the wire 400 contacts with the work 900 should be limited to some mA for preventing an electric shock. Although the resistance value required for this purpose is determined by the DC voltage applied to the torch cable 2100, the resistance 1400 should be greater than some tens K.OMEGA. generally. Due to this greater resistance, the adjustable range for the reference voltage of the adjustable resistor 1600 is very narrow. In addition to this narrow adjustable range, the various factors also affect the overall impedances. Then, the wire contact detector 2600 can not detect the "wire contact" correctly.
Referring back to FIG. 7, a switching mechanism provided to solve this problem is shown. This switching mechanism uncouples the internal resistance 1300 from the welding power supply circuit only when a high DC voltage is applied between the work 900 and welding wire 400. In welding operation, the internal resistance 1300 should be connected and disconnected so frequently. Therefore, a relay contact can not used for uncoupling use, because the relay contact will wear out so rapidly, resulting in a shorter service life of the welding system itself.
To avoid such shorter service life due to relay contact, this switching includes the select switch 2901 for uncoupling the internal resistance 1300 from the power source unit 200 during the contact detection. Specifically, only when a high DC voltage is applied between the work 900 and welding wire 400, the select switch 2901 actuates the selector S to remove from the internal resistance 1300 (line 2300). It is to be noted that this switching mechanism requires the signal line 2300 instead of the relay contact.
Therefore, the operator is required to additionally work to wire this signal line 2300 at site when installing the arc welding system. Such work is originally unnecessary if relay is incorporated in the system for the use of uncoupling the internal resistance 1300.
Furthermore, in such a welding robot system with an arc welding sensor (not shown in FIG. 7), a resistor used for the detection of welding voltage, incorporated in the arc welding sensor, is connected to a positive terminal 2700 and a negative terminal 2800 of the power source unit 200. Thus, a circuit is formed by the resistance 1300, causing a voltage drop even when the wire 400 and work 900 are not in contact with each other. In this case, the welding sensor works in error.
It is also to be noted that one end of the internal resistance 1300 is connected to the select switch 2901 that is connected to the torch line 2100, as shown in FIG. 7. Therefore, when the selector S is mis-operated, high voltage of welding power may be connected to the positive side of the system during the welding operation, it is very dangerous.
Furthermore, the wire contact detector 2600 should be independently provided, separately from the controller 100, the welding power source unit 200, and the robot 700, as shown in FIG. 7. The welding system itself needs broader area for installation and operation, and results in increased manufacturing cost. It is needless to say that it will take more cost if this detector 2600 is forcibly incorporated in any of other parts for the space saving.
In FIG. 9, the welding start signal Sws and the wire contact detection start signal Swc observed in a conventional arc welding system are shown. Specifically shown, a high level period Pa of the wire contact detection start signal Swc is completely included in a high level period Pw of the welding start signal Sws. This means that the wire contact detection voltage is applied in error to the lines where the welding power current are flowing, causing a hazardous situations such as burning of components of the welding system.